Electron beam pumped non-c-plane UV emitters

ABSTRACT

An ultraviolet (UV) radiation emitting device includes an epitaxial heterostructure comprising an AlGaInN active region. The AlGaInN active region includes one or more quantum well structures with Al content greater than about 50% and having a non-c-plane crystallographic growth orientation. The AlGaInN active region is configured to generate UV radiation in response to excitation by an electron beam generated by an electron beam pump source.

RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 15/600,569, filedMay 19, 2017, which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under Contract No.HR0011-15-C-0025 DARPA-LUSTER Program awarded by DARPA. The Governmenthas certain rights in this invention.

TECHNICAL FIELD

This application relates generally to devices that emit ultravioletradiation and to systems and methods pertaining to such devices.

BACKGROUND

Ultraviolet (UV) emitting devices are of considerable interest forapplications that include water purification, analytical devices formedical and biotechnology fields, UV curing, and currency screening,among other applications. Light emitting devices that emit in spectralranges suitable for these and other applications can be fabricated basedon a variety of material systems, including group III-V and II-VIbinary, ternary, and quaternary compounds and alloys and variouscombinations thereof.

SUMMARY

Some embodiments are directed to a device that emits ultraviolet (UV)radiation through spontaneous emission. The UV radiation emitting deviceincludes an epitaxial heterostructure comprising an AlGaInN activeregion. The active region includes one or more quantum well structureswith Al content greater than about 50% having a non-c-planecrystallographic growth orientation, The AlGaInN active region isconfigured to generate UV radiation in response to excitation by anelectron beam from an electron beam pump source.

Some embodiments are directed to a device that emits ultraviolet (UV)radiation through stimulated emission. The UV radiation emitting deviceincludes an epitaxial heterostructure comprising an AlGaInN activeregion. The active region includes one or more quantum well structureswith Al content greater than about 50% and having a non-c-planecrystallographic growth orientation. The AlGaInN active region isconfigured to generate UV radiation in response to pumping by anelectron beam. The device includes a first reflector and a secondreflector, wherein the active region is disposed between the firstreflector and the second reflector.

Some embodiments are directed to a method of operating a UV radiationemitting device. The method includes electron beam pumping an activeregion of an epitaxial III-N heterostructure having a non-c-planecrystallographic orientation. UV radiation is generated in the activeregion in response to pumping by the electron beam. The UV radiation hasa wavelength less than about 250 nm.

Some embodiments are directed to a method of making an UV radiationemitting device. A bulk AlN boule is sliced to provide a substratehaving growth surface with a non c-plane crystallographic orientation. Aheterostructure is epitaxially grown on the AlN substrate. Theheterostructure comprises a UV emitting active region and has thenon-c-plane crystallographic orientation of the growth surface. In someembodiments, an epitaxial AlN layer may be epitaxially grown on thegrown surface of the bulk AlN boule before the heterostructurecomprising the UV emitting region is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1F show crystallographic planes of hexagonal materials;

FIG. 2 shows a cross sectional diagram of a UV emitting device that canproduce spontaneous UV emission in accordance with some embodiments;

FIGS. 3A through 3C show cross sectional diagrams of UV emitting devicesthat can produce spontaneous UV emission in accordance with someembodiments;

FIGS. 4A through 4C show the e-beam penetration into the heterostructureat e-beam energies of 5 keV, 10 keV, and 15 keV, respectively;

FIG. 4D is a graph of simulated electron penetration depth as a functionof electron beam energy in eV for vertical incidence of the electronbeam;

FIG. 5 shows a cross sectional diagram of a UV emitting laser devicethat can produce stimulated UV emission in accordance with someembodiments;

FIGS. 6, 7, and 8 show approaches for the formation of a laser devicehaving a non-c-plane oriented active region in accordance with someembodiments;

FIG. 9 is a side-view of a semiconductor laser structure according toone embodiment of the present disclosure;

FIG. 10 is a plot of refractive index and field profile versus thicknessfor a sample device according to an embodiment of the presentdisclosure;

FIG. 11A is a plot of refractive index and energy deposition profileversus thickness for the sample device of FIG. 10;

FIG. 11B is a plot of absorption loss against top cladding thickness forthe sample device of FIG. 10;

FIG. 12 is a plot of refractive index and field profile versus thicknessfor a sample device according to another embodiment of the presentdisclosure;

FIG. 13A is a plot of refractive index and energy deposition profileversus thickness for the sample device of FIG. 12;

FIG. 13B is a plot of absorption loss against top cladding thickness forthe sample device of FIG. 12;

FIG. 14 is a side-view of two semiconductor laser structures, includingconduction path metallization, according to an embodiment of the presentdisclosure;

FIG. 15 is a cut-away side view of a light emitting device having asubstrate and via connection to a discharge structure, according to anembodiment of the present disclosure;

FIG. 16 is a top-view of the two semiconductor laser structures,including conduction path metallization, according to the embodiment ofFIG. 14;

FIG. 17 is a high-resolution ω-2Θ x-ray scan of 10× AlGaN multiplequantum well heterostructure on semipolar (20-21) bulk AlN substrate inaccordance with some embodiments;

FIG. 18 is an atomic force microscopy image of a 10× AlGaN multiplequantum well heterostructure grown on (20-21) bulk AlN substrate inaccordance with some embodiments;

FIG. 19 shows the temperature-dependent photoluminescence spectrarecorded from a semipolar (20-21) AlGaN multiple quantum wellheterostructure in accordance with some embodiments;

FIG. 20 shows a graph of the normalized temperature-dependentphotoluminescence intensity where values for the intensities are gainedfrom integrating the spectra of FIG. 10; and

FIG. 21 shows polarization-resolved photoluminescence spectra fromsemipolar AlGaN multiple quantum wells grown on (20-21) bulk AlNsubstrate in accordance with some embodiments.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DESCRIPTION

The efficiency of UV emitting devices that are grown on c-planesubstrates or templates drops by several orders of magnitude as theemission wavelength becomes shorter than about 250 nm. The drop inefficiency occurs because the radiation is mainly transverse-magnetic(TM) polarized at the shorter wavelengths and cannot be easily extractedout of the wafer plane. Embodiments described herein involve structuresin which the crystal orientations of the substrate, and thus theorientation of the light emitting layers, e.g., quantum wells (QWs)grown on the substrate, are rotated to a non-c-plane orientation.Rotating the crystal orientation of the substrate and QWs can enhanceextraction of UV radiation, particularly for UVC radiation between 200nm and 290 nm.

Some embodiments discussed herein involve approaches that enhance theefficiency of light extraction from vertically emitting or edge emittingUV sources based on the AlGaN material system. The approaches discussedbelow involve the use of non-c-plane oriented AlGaN quantum wells thatmay be epitaxially grown on a non-c-plane oriented surface of an AlNsubstrate.

Another major challenge for UV wavelength emitters is the p-typeconductivity of these high band gap materials needed for UV radiation.Today's device efficiencies suffer from either high light absorptionlosses when p-GaN is used as hole injection layer or high voltages andreduced carrier injection when p-AlGaN materials are implemented to gainat least partially UV transparency. Some embodiments discussed hereinuse electron beam (e-beam) excitation for the creation of electron-holepairs and decreased light absorption losses.

FIGS. 1A through 1F show crystallographic planes of hexagonal materialssuch as GaN and AlN. FIG. 1A illustrates the polar plane (0001),referred to as the c-plane; FIG. 1B illustrates the non-polar (1100) or(1-100) plane, referred to as the m-plane; and FIG. 1C illustrates thenon-polar (1 120) or (−1-120) plane, referred to as the a-plane. FIGS.1D through 1F show semi-polar planes (1122) or (11-22), (2021) or(20-21), and (202 1) or (20-2-1). Other common semi-polar crystal planesthat could be used for embodiments disclosed herein include (10-11),(10-1-1), (11-2-2), (10-13), (10-1-3), (30-31), and (30-3-1), andothers. High quality c-plane AlGaN structures can be grown on c-planesubstrate surfaces, but the UV emission is only TE polarized for longeremission wavelength and the device is plagued by extraction issuesdiscussed above. AlGaN structures can be grown on non-polar m- anda-plane substrate surfaces. However, epitaxial growth conditions aremore demanding for producing materials with featureless surfaces thatare free of structural defects. Some embodiments described herein aredirected to devices with AlGaN or AlGaInN QWs grown on surfaces withsemi-polar orientations. As discussed below, growth on semi-polarorientations, such as (20-21) and (20-2-1) planes, were shown to producesmooth surfaces and sharp heterostructure interfaces needed for highquality UV emitters.

FIG. 2 shows a cross sectional diagram of a UV emitting device 200 thatcan be used to produce spontaneous UV emission 221 in accordance withsome embodiments. The device includes an epitaxial heterostructure 220comprising a UV emitting active region. For example, the UV emittingactive region may comprise one or more quantum well (QW) structures thatare grown to have a non-c-plane crystallographic orientation. The activeregion is configured to emit radiation in response to pumping by anelectron beam 231 produced by an electron beam source 230. In someembodiments, the UV active region includes AlGaN or AlGaInN with Alcontent greater than 50% such that the UV radiation emitted by theactive region is less than about 250 nm. Portions of the device 200 maybe disposed within a vacuum chamber 280 in some embodiments.

In some embodiments, the active region may comprise several quantumwells, e.g., 1 to 50 quantum wells, having Al composition of about 70%.The thickness of the quantum wells can be in a range of about 0.5 nm toabout 5 nm, or in a range of about 1 nm to about 3 nm, for example. Eachquantum well is disposed between barrier layers. In some embodiments,the Al composition of the barrier layers is in a range of about 80% toabout 99%. The active region including the quantum wells and barrierlayers may have a thickness of between about 200 nm to about 1000 nm orbetween about 400 nm to about 800 nm in some embodiments.

To further facilitate current flow through the heterostructure 220, oneor more heterostructure layers may be doped with n-type dopants,increasing the electrical conductivity of the layers. Theheterostructure layers may be doped n-type by incorporating impuritiessuch as Si, Ge, and Sn. For example, one or more quantum wells and/orbarrier layers may be doped with an n-type dopant at a level of about10¹⁷ to 10²⁰/cm³ to achieve a conductivity in the doped layers betweenabout 0.01 (Ωcm)⁻¹ and 10⁴ (Ωcm)⁻¹. For example, at least a portion ofthe heterostructure layers may be n-doped with Si at a level greaterthan about 10¹⁷/cm³. The impurity concentration is typically less than3% of the total atomic concentration in the material.

In some embodiments, the heterostructure 220 may have a non-polarcrystallographic orientation, e.g., m-plane or a-plane orientation. Insome embodiments, the heterostructure may have a semi-polarcrystallographic orientation. For example, suitable semi-polarorientations include the (10-11), (11-22), (20-21), (20-2-1), (10-11),(10-1-1), (11-2-2), (10-13), (10-1-3), (30-31), and (30-3-1),crystallographic orientations.

FIGS. 3A through 3C show cross sectional diagrams of UV emitting devices300A, 300B, 300C that can be used to produce spontaneous UV emission inaccordance with some embodiments. The epitaxial heterostructure 320includes a non-c-plane oriented active region that is grown on and takesthe orientation of a substrate 310 with the non-c-plane orientation. Insome embodiments, the substrate 310 comprises a bulk AlN substrate thathas been sliced from a bulk AlN boule or otherwise formed to provide anon-c-plane epitaxial growth surface. The substrate 310 may include anepitaxial AlN layer grown, e.g., by metal-organic vapor phase epitaxy(MOVPE) on the bulk AlN substrate. In some embodiments, the activeregion includes AlGaN with Al content greater than 50% such that the UVradiation emitted by the active region is less than about 250 nm. Thesurface roughness of the heterostructure 320 may be less than about 0.35nm as measured by atomic force microscopy.

As illustrated by device 300A of FIG. 3A, the substrate 310 may beremoved entirely, as indicated by arrow 399, or may be mechanicallyand/or chemically thinned to a substrate remnant 311 having a thicknessless than about 100 μm or less than about 50 μm or even less than 20 μm,for example. After removal or thinning of the substrate, the backside ofthe heterostructure (or the remaining surface of the substrate remnant)may be roughened for enhanced light extraction.

In some embodiments, the AlGaN QWs may have a thickness between 0.5 nmand 5 nm, or between 1 nm and 2 nm. Barrier layers that separate the QWscan have barrier thicknesses between 5 and 50 nm. The optimal activezone thickness depends on e-beam energy, for example between 5 to 30keV. FIGS. 4A through 4C show the e-beam penetration into theheterostructure at e-beam energies of 5 keV, 10 keV, and 15 keV,respectively. FIG. 4D is a graph of simulated electron penetration depthin μm as a function of electron beam energy in eV for vertical incidenceof the electron beam in a sample comprising 47 Al_(0.7)Ga_(0.3)N/AlNQWs/barriers on bulk AN.

As shown in FIG. 3B, the UV emitting device 300B can include a heat sink340 disposed on the heterostructure 320. Suitable heatsink materialsinclude diamond, copper, copper-tungsten, aluminum, AlSiC, and/or othermaterials or material composites. In the case of non-transparent heatsink or heat spreader materials and bottom light emission a (beveled)hole should be included in the heat sink. If the semiconductor film israther thin it needs to be mounted on a transparent heat spreadermaterial first (e.g., diamond, sapphire, quartz, etc.)

As shown in FIG. 3C, the UV emitting device 300C may include one or morecontacts 350 electrically coupled to the heterostructure 320 andconfigured to provide a current path that discharges electrons arisingfrom the electron beam pumping of the heterostructure 320. In someembodiments, the contact 350 is a single contact 350 that can include anaperture 351 which exposes the heterostructure to the electron beam 231.In some embodiments, the UV emitting device can include multiplecontacts coupled to the same electrical potential to discharge theelectrons arising from the electron beam pumping of the heterostructure320. In some embodiments the contact can be a continuous metal film thatis thin enough to be penetrated by the e-beam and also acts as anoptical reflector. For example, the contact 350 may comprise Al whichshows a high reflectivity in the deep-UV.

In some embodiments, a UV emitting device may be a vertically emittinglaser device having a non-c-plane oriented active region which isconfigured for generation of UV stimulated emission. FIG. 5 shows across sectional diagram of a UV vertically emitting laser device 500that can produce stimulated UV emission 521 in accordance with someembodiments. The device 500 includes an epitaxial heterostructure 520comprising a UV emitting active region having a non-c-planecrystallographic orientation. The UV emitting active region may compriseone or more quantum well structures that are grown to have thenon-c-plane crystallographic orientation. The heterostructure 520 isdisposed between. The active region is disposed between first and secondreflectors 561, 562 and is configured to emit laser radiation inresponse to pumping by an electron beam 231. In some embodiments, the UVactive region includes AlGaN with Al content greater than 50% such thatthe UV radiation emitted by the active region is less than about 250 nm.Additional information regarding structures for electron beam pumpedvertical cavity surface emitting lasers is described in commonly ownedU.S. Pat. No. 9,112,332 which is incorporated herein by reference.

As previously discussed, the heterostructure 520 may have a non-polarcrystallographic orientation, e.g., either an m-plane or an a-planeorientation. In some embodiments, the heterostructure 520 may have asemi-polar crystallographic orientation such as the (10-11), (11-22),(20-21), (20-2-1), or other crystallographic orientations.

FIGS. 6 and 7 show approaches for the formation of a laser device havinga non-c-plane oriented active region. In some embodiments, shown in thetop cross sectional diagram of FIG. 6, reflector 661 is a distributedBragg reflector (epi-DBR) that is epitaxially grown on the non-c-planeoriented substrate 610. The epi-DBR 661 comprises a series ofalternating semiconductor layers disposed on the substrate 610 that takeon the non-c-plane orientation of the substrate 610. For example, ifnon-c-plane oriented AlN is used as the substrate 610, then an epi-DBRmay comprise alternating layers of non-c-plane oriented AlGaN and AlNgrown on the non-c-plane oriented AlN substrate 610. Using an epi-DBRhelps to promote heat dissipation from the active region because thesemiconductor layers of the epi-DBR can provide some heat sinking. Anon-c-plane oriented heterostructure 620, including the active region isepitaxially grown on the non-c-plane oriented epi-DBR 661.

As shown in the middle cross sectional diagram of FIG. 6, after thenon-c-plane oriented epi-DBR 661+ heterostructure subassembly 605 isformed, the substrate 610 may optionally be fully or partially removed,for example, using a laser liftoff process (LLO), mechanical polishingand/or dry/wet chemical etching. In some configurations, a thin remnantof the substrate may remain after the removal.

As shown in the bottom cross sectional diagram of FIG. 6, in someconfigurations the DBR+heterostructure subassembly 605 can be bondedepi-DBR side down to a heat sink 640 to enhance heat dissipation fromthe active region. The heat sink 640 may comprise a metal, metal-alloyor other materials having sufficient thermal conductivity, e.g.,diamond, copper, copper-tungsten, aluminum, AlSiC, and/or othermaterials or material composites. One or more contacts 650 fordischarging electrons from the active region can be deposited over theheterostructure 620.

Another approach for the formation of a laser structure, shown in FIG.7, involves the use of a first DBR 761 comprising a dielectric layerswhich are deposited on the backside of a non-c-plane orientedepitaxially grown heterostructure 720. For example, the dielectric DBR761 may comprise pairs of SiO₂/Al₂O₃, SiO₂/Y₂O₃ or SiO₂/HfO₂. Dielectricmaterials used to form a dielectric DBR can have higher refractive indexcontrast than semiconductor materials. Thus, for the same reflectivity,a dielectric DBR can have fewer layer pairs when compared to the numberof semiconductor layer pairs of an epitaxially grown DBR. However,dielectric materials have lower thermal conductivity which may be afactor for devices that need heat dissipation from the active region.

As shown in the cross sectional diagram at the top of FIG. 7, anon-c-plane oriented heterostructure 720 comprising a non-c-planeoriented active region configured to emit UV radiation is epitaxiallygrown on substrate 710. The substrate 710 is fully or partially removedfrom the heterostructure 720 as shown in the middle cross sectionaldiagram of FIG. 7. After removal of the substrate, a dielectric DBR 761is deposited on one surface of the heterostructure 720, e.g. thebackside surface which is the surface of initial epitaxial growth of theheterostructure 720. If a remnant of the substrate remains, thedielectric DBR 761 is deposited on the substrate remnant.

One or more contacts 750 are disposed on the heterostructure surfaceopposite the dielectric DBR as shown in the bottom cross sectionaldiagram of FIG. 7. In the implementation shown in FIG. 7, theDBR+heterostructure subassembly 705 can be arranged DBR side down on aheat sink 740 that provides dissipation of heat generated in the activeregion.

Returning now to FIG. 5, according to some embodiments, one or more ofthe first and second reflectors 561, 562 may be external reflectors. Agap separates an external reflector from the heterostructure 520 and/orother layers disposed on the heterostructure 520. An external reflectormay include an epitaxial DBR and/or a dielectric DBR as discussed above.In some embodiments, the second DBR may comprise an epi-DBR ordielectric DBR disposed on the heterostructure as illustrated by FIG. 7.

The cross sectional diagram at the top of FIG. 8 shows a non-c-planeoriented heterostructure 820 comprising a UV emitting active region. Thenon-c-plane oriented heterostructure has been epitaxially grown on anon-c-plane oriented substrate which is subsequently completely removedor thinned to a remnant and is not shown in FIG. 8. After removal of thesubstrate, a dielectric DBR 861 is deposited on one surface of theheterostructure 820, e.g. the backside surface which is the surface ofinitial epitaxial growth of the heterostructure 820. If a remnant of thesubstrate remains, the dielectric DBR 861 is deposited on the substrateremnant. Alternatively, a non-c-plane oriented epi-DBR may be grown onthe substrate prior to growth of the non-c-plane orientedheterostructure 820.

One or more contacts 850 are disposed on the heterostructure surfaceopposite the DBR 861 as shown in the bottom cross sectional diagram ofFIG. 8. In the implementation shown in FIG. 8, the DBR+heterostructuresubassembly 805 can be arranged DBR side down on a heat sink 840 toprovide for dissipation of heat generated in the active region.

A second DBR 862, e.g., an epi-DBR or dielectric DBR, may be epitaxiallygrown or deposited such that the second DBR 862 is disposed within theaperture 851 of the patterned contact 850.

In some embodiments, a UV emitting device may be an edge emitting laserdevice having a non-c-plane oriented active region. The edge emittinglaser device may be configured for generation of UV stimulated emission.In some embodiments the edge emitting devices are grown on a substrate,e.g., AlN having a non-polar orientation along the (1100) or (1-100)plane, referred to as the m-plane. In some embodiments the surface ofthe substrate may be oriented along the non-polar (1 120) or (−1-120)plane, referred to as the a-plane. The surface of the substrate may beoriented along a semi-polar plane such as (1122) or (11-22), (2021) or(20-21), and (202 1) or (20-2-1) in some embodiments. Other commonsemi-polar crystal planes that could be used in embodiments disclosedherein include (10-11), (10-1-1), (11-2-2), (10-13), (10-1-3), (30-31),and (30-3-1), and others. The layers above the substrate, including thecladding and active layers, take on the semi-polar or non-polarorientation of the substrate.

FIG. 9 shows a cross sectional diagram of a UV edge emitting laser 10 inaccordance with some embodiments. Device 10 comprises a semiconductorlaser structure 12 and electron-beam pump source 14 according to oneembodiment of the present disclosure. Semiconductor laser 12 comprises anon-polar or semi polar substrate 16. In some embodiments, the substrate16 may comprise bulk AlN. Above substrate 16 is formed a lower claddinglayer 18 of Al_(x)Ga_(1-x-y)In_(y)N where x is between 0.6 and 1 and yis between 0 and 0.03, for example n-Al_(0.74)Ga_(0.26)N.

A lower waveguide 20 may be formed over lower cladding layer 18 ofAl_(z)Ga_(1-z-y)In_(y)N where z is between 0.5 and 1 and y is between 0and 0.03, and z<x. For example, lower waveguide 20 may comprisen-Al_(0.7)Ga_(0.3)N (e.g., 40 nm thick).

An active layer 22, such as a multiple quantum well heterostructure(MQW) or double heterostructure (DH) region may be formed over lowerwaveguide 20. Active layer 22, in the case of a MQW, may comprise pairsof Al_(x)Ga_(1-x)N/AlyGa_(1-y)N of varying thickness (e.g., 5 layerpairs of Al_(0.57)Ga_(0.43)N/Al_(0.62)Ga_(0.38)N, 5.4 nm and 9.6 nm,respectively). In general, active layer 22 may comprise at least onelayer of Al_(u)Ga_(1-u-v)In_(v)N where v is between 0 and 0.03, and0.4<u<z. In the case of a MQW, in general the barrier may beAl_(s)Ga_(1-s-t)In_(t)N, where 0.4<u<Z and s>u+0.04, and t is between 0and 0.03. In such a case, the quantum well thickness may be between 1and 6 nm and the barrier thickness may be between 3 and 20 nm.

An upper waveguide 24 may be formed over active layer 22, comprising,for example, n-doped AlGaN (such as 40 nm of n-Al_(0.7)Ga_(0.3)N),undoped AlN, and so on.

Optionally, upper cladding layer 26 may be formed over upper waveguide24. (Optional layers, elements, and features for various embodiments maybe indicated with dashed outlines.) When fabricated without an uppercladding layer, upper waveguide 24 is formed to be relatively thick, onthe order of about 200 nm for example. When fabricated with uppercladding layer 26, layer 26 may comprise n-doped AlGaN (such as havingat least 70% Al, for example n-Al_(0.78)Ga_(0.22)N, 220 nm thick). Insome embodiments, such as at very high Al concentration, the uppercladding layer 26 may be undoped. In some embodiments, at least one ofthe lower cladding layer, lower waveguide layer, light emitting layer,upper waveguide layer, or upper cladding layer is doped n-type.

Optionally, in certain embodiments, at least one of the lower claddinglayer, lower waveguide layer, light emitting layer, upper waveguidelayer, and upper cladding layer is a short-period superlattice. Inaddition, in certain embodiments, at least one of the lower claddinglayer, lower waveguide layer, upper waveguide layer, or upper claddinglayer may be a monotonically-varying alloy-compositional gradient, withthe lower band gap composition of the graded layer nearest the lightemitting layer.

Also optionally, a contact layer 28 may be provided over upper claddinglayer 26. Contact layer 28 may comprise n-GaN as one example. A suitableohmic metal layer 29 may be formed over contact layer 28 (such as 30 nmof Ti) to permit conduction of charge from the laser device, discussedfurther below.

It will be appreciated that at relatively high thicknesses of activelayer 22, active layer 22 may itself provide a wave guiding function,obviating the need for a separate lower waveguide 20 and/or upperwaveguide 24, and otherwise providing a light guiding layer. Thus, incertain embodiments lower and upper waveguides form a light guidingstructure within which a light emitting layer is disposed. In otherembodiments, the thickness and other characteristics of the lightemitting layer provide light guiding at upper and lower margins, therebyalso forming a light guiding structure within which a light emittinglayer is disposed.

Electron-beam pump source 14 is disposed to be over, and in someembodiments, spaced apart from a top surface of semiconductor laser 12.Electron beam (“e-beam”) pump source 14 is connected to a drivingvoltage such that it produces a line-pattern (e.g., 12 μm×500 μm)electron beam in a direction toward and into the top surface ofsemiconductor laser 12.

The structure described above has been built and modeled. Emissionwavelengths for such a structure may be obtained, depending uponmaterials and compositions, for example in the range of 200 nm to 365nm. FIG. 10 is an illustration of the modeling of the structure for atarget emission wavelength at λ=265 nm. For the model, the structureincludes a 220 nm thick n-Al_(0.78)Ga_(0.22)N top cladding layer, a 5 nmthick n-GaN contact layer, and a 30 nm thick Ti metal topcontact/discharge layer. As can be seen from FIG. 9, the refractiveindex for the various layers is shown, as is the optical mode (i.e.,e-field of the lasing mode) profile. In the ideal case, the carriergeneration peaks at the center of the MQW, in order to most efficientlygenerate photons therein. The model of FIG. 10 predicts a comparativelyhigh Γ, on the order of 25%, and a comparatively low loss of less than 1cm⁻¹.

FIG. 11A is a graph of the modeling of the structure of FIG. 9, showingcalculated energy deposition profile versus depth into the semiconductorlaser structure for two selected e-beam energies. As can be seen fromFIG. 10A, for 10 keV and 12 keV, about 32 percent and 30 percent of thetotal energy, respectively, is deposited into the MOW region andwaveguide layers (upper and lower). The rate of electron-hole generationat a given depth is proportional to the energy deposition at that depth.The more energy that is deposited into these regions, the higher theexpected laser output power.

The effect of cladding layer thickness on absorption losses for thisstructure is illustrated in FIG. 11B. It can be seen from FIGS. 11A and11B that a thicker cladding layer limits the depth of the peak of theenergy deposition profile yet provides lower absorption loss, andconversely a thinner cladding layer permits a deeper peak of the energydeposition profile (e.g., closer to the active region) yet results inhigher absorption loss. From data such as provided in FIGS. 11A, 11B,design parameters may be chosen as a function of the specificapplication of the teachings of the present disclosure to permitbalancing and ultimately optimizing both the energy deposition profileof the electron beam and the absorption losses induced mainly by the topsurface layers (contact layer, top metallization).

The results shown in FIGS. 10 and 11A, 11B support the operability andadvantages of a laser structure, such as that described above, thatoperates without an upper p-doped layer. Again, one significantadvantage provided thereby is elimination of a p-doped layer with highelectrical conductivity and low absorption losses that is verychallenging to fabricate for high band gap materials.

Using an electron beam as an excitation source takes advantage of thefact that generation of carriers by electron beam means that carriergeneration and injection does not rely on a p-/n-junction. This obviatesthe challenge of forming a highly conductive (p-type) material in highband gap semiconductors that is able to carry the current densitiesnecessary for laser emission. Thus, no p-type doping of an uppercladding layer is required. In addition, carrier injection by e-beam iscomparatively deep, extending beyond the top most MQW layers. Thishomogeneous carrier injection supports an increased number of quantumwells in the MQW layer than typically is achieved in a laser diode.Higher gain and improved device performance are thereby provided.

E-beam pumping produces a net charge within the laser structure. Thus,the laser heterostructures and the device architectures disclosed hereininclude features to allow effective discharge of the device, e.g., itincludes conductive layers that are n-doped (and may include p-dopedlayers), metal films and/or contacts and connections to ground or theanode of the e-beam source. In the example above, an n-type uppercladding and metal ohmic contact are provided over the MQW, solely forthis discharge function. However, it will be understood that this issimply one example of a family of structural configurations in which anupper p-type cladding is obviated bye-beam pumping. Many variations arecontemplated herein.

For example, FIG. 12 shows an alternative semiconductor laser design forelectron beam pumped operation at a target emission wavelength of A=265nm, and FIG. 13A shows calculated e-beam energy deposition profile forthat design. FIG. 13B illustrates the effect of cladding layer thicknesson absorption losses for this structure. The structure modeling in FIGS.12 and 13A, 13B is similar to that of FIGS. 10 and 11A, 11B, with theexception that the structure of FIGS. 12 and 13A, 13B has an undoped 120nm thick AlN top cladding layer and a lack of any contact layer andupper ohmic metal layer. Noteworthy is the result that lower absorptionlosses may be obtained for a thinner top cladding layer as compared tothe structure of FIGS. 10 and 11A, 11B. The energy profile peak maytherefor occur closer to the active region. The lack of upper contactlayers means that discharge of the device is provided not via the topsurface, but instead through the n-AlGaN lower cladding layer and/orlaterally via mesa side contacts, as an option.

The laser heterostructure in the prior example was designed to allowdischarge via its top surface. Therefore the AlGaN composition of thecladding layer was limited to provide sufficiently high conductivity(e.g., the AI-composition is 78%). To avoid high absorption lossesthrough the GaN cap layer and/or metal contact, the cladding layer inthat example was chosen to be relatively thick, on the order of 220 nm.In the present example, however, there is no concern with regard toconductivity of the upper cladding layer, so it may be entirelynon-conductive AlN. Furthermore, by reducing the thickness of the AlNupper cladding later, carrier generation and confinement within the MQWactive region is improved. Thus, the upper cladding layer was selected,as an example here, to be on the order to 120 nm thick.

In order to provide a discharge path for the device illustrated in FIG.12, etching is performed to form a mesa structure, with the lowercladding layer exposed for electrical contact. Metal may be deposited onselected portions of the exposed surfaces, either at the bottom of theetched region adjacent the device or the sidewall of the device, orboth, for the requisite conduction path. Again, no GaN contact layer ortop metal is needed. The waveguide simulations produce a comparativelyhigh Γ, on the order of 25%, and a comparatively low loss of less than 1cm⁻¹.

The energy deposition profiles of the excitation source are shown inFIG. 13A for excitation energies of 8, 10, and 12 keV. This produced 52,41, and 28% of the total energy is deposited into the active zone andwaveguide layers for 8, 10, and 12 keV, respectively. The absolutedeposited energy in the active zone and waveguide layers, with the samecurrent for the different excitation energies, is very similar for 8 and10 keV and about 20% less for 12 keV.

An individual device or array of devices may be formed on a substratehaving a non-polar or semi-polar orientation and having the layercomposition illustrated in FIG. 14. A structure may be formed with sucha layer composition, and then etched to form individual devices. Twosuch devices 30, 32 are shown in FIG. 14, separated by an etched region34. A dry etch, for example using chemically assisted ion beam etching(CAIBE) or inductively-coupled plasma reactive ion etching (ICP-RIE),facet cleaving, or other processes known in the art may be used to formboth the etched region 34 as well as the laser facets. Etching mayproceed into the lower cladding layer 18, exposing that layer. A contactmetal 40 (for example, a layered structure of Ti(20 nm)/Al(100 nm)/Ni(45nm)/Au(60 nm) annealed at about 900° C. for roughly one minute, whichforms a functional ohmic contact to the n-type AlGaN) may then be formedover portions of devices 30, 32, and within etched region 34 to form aconduction path for discharging the devices (e.g., tied to ground).Optionally, sidewall passivation 42 may be formed on the sidewalls ofdevices 30, 32 prior to forming contact metal 40.

While the previous embodiment forms a discharge path using a depositedmetal in etched regions between devices, in other embodiments, directcontact to the ohmic metal layer 29 may be made to each device, orgroups of devices, for discharging. As mentioned, contact layer 29 isrelatively thin, on the order of 30 nm for a material such as Ti, suchthat the electron beam may easily penetrate into the active region ofthe device. Similarly, a backside metal layer may be provided forelectrical and thermal discharge. One such device 70 is illustrated inFIG. 15. Device 70 comprises a substrate 72 having a via 74 formedtherein, and a conductive discharge structure 76 disposed on a firstsurface. A light emitting device 78 is disposed on a second surface ofsubstrate 72 opposite the surface on which the light emitting device 78is disposed. Via 74, or more precisely, conductive material disposed invia 74, electrically interconnects conductive discharge structure 76 anddevice 78 such that charge that may accumulate in device 78 due toelectron beam pumping from electron beam pump 80 may be dischargedthrough structure 76, such as to ground potential. Many other dischargestructures are contemplated hereby, including one or more conductiveintermediate layers, sidewalls, and so on.

Following the formation of devices 30, 32, individual electron beamsources may be formed or disposed over each device. According to oneembodiment, prefabricated electron beam sources are selected to have abeam length comparable to the length of the resonance cavity (betweenfacets 50, 52 of device 30 and facets 54, 56 of device 32), for exampleon the order of 300-500 μm. Indeed, the length of the resonator isprimarily limited by the output format of the electron beam source. FIG.16 illustrates a top-view of devices 30, 32, with the position of anelectron beam on each device indicated by lines 44, 46, respectively.

The resulting device may be operated in vacuum. According to certainembodiments, the output power density may be in the range of 50 kW/cm²to about 1 to 2 MW/cm². The acceleration voltage may be on the order of10 kV. The spot size may be on the order of 12×500 microns. That is,current is on the order of 0.6 mA to 6-12 mA, with a higher currentproducing a higher the output power. Again, these ranges are merely oneset of examples, and other ranges are also contemplated by the presentdisclosure. Additional information regarding e-beam pumped edge-emittingdevices is described in U.S. Pat. No. 8,964,796 which is incorporatedherein by reference.

EXAMPLE

AlGaN multiple quantum wells (MQWs) were grown on semipolar (20-21) ANto mitigate the issues related to the polarization switching forshort-wavelength deep-UV optical emitters. Structural properties wereaccessed by x-ray diffraction and atomic force microscopy (AFM) whilethe emission behavior was monitored by photoluminescence (PL) studies.

The single crystal AlN substrates used in these studies were fabricatedfrom AlN boules grown by physical vapor transport and were nominally(20-21) oriented with less than 1° in miscut. After slicing, thesubstrates were chemo-mechanically planarized (CMP) and have a typicalroot mean square (rms) surface roughness of about 0.1 nm as measured byAFM. The structural quality of the substrates were confirmed by x-rayrocking curve measurements which reveal full width at half maximum(FWHM) values as low as 37 arc sec for both the (002) and (102)reflections. The dislocation density was less than 10⁵ cm⁻².

The AlGaN heterostructures were grown by metal-organic vapor phaseepitaxy (MOVPE) with conventional pre-cursor sources. The multiplequantum wells (MQWs) active zone consisted of 10 pairs ofAl_(x)Ga_(1-x)N/Al_(y)Ga_(1-y)N QWs with a thickness of about 3 and 10nm, and x=0.69, y=0.90, respectively. FIG. 17 compares simulated 1701and measured 1702 plots of ω-2Θ x-ray diffraction scan of the (20-21)semipolar AlGaN MQW stack. As can be seen, higher-order Pendelloesungfringes can be resolved, which imply sharp and abrupt interfaces betweenthe individual layers of the heterostructure. Good agreement betweenmeasurement and simulation is also evident.

To further assess the structural properties atomic force microscopy(AFM) images were taken from the sample surface. FIG. 18 is an AFM imageof the sample surface. As shown in FIG. 18 relatively smooth layerscould be realized with an rms surface roughness of less than 1 nm oreven less than 0.35 nm for a 5 μm×5 μm scan.

The optical properties were assessed by temperature- andpolarization-dependent photoluminescence measurements by using a pulsed193 nm ArF excimer laser as excitation source. The emission was recordedperpendicular to the sample surface without a polarizer in the opticalpath for the temperature-dependent measurements. FIG. 19 shows a seriesof photoluminescence (PL) spectra recorded between 8K and roomtemperature 295K. A clear QW emission was observed at about 237 nm atlow temperature which shifts to longer wavelength with increasingtemperature. The rather pronounced decrease in emission intensity withincreasing temperature can be ascribed to the presence of non-radiativerecombination channels that dominate the recombination process atelevated temperatures. An upper value for the internal quantumefficiency (IQE) of about 7.5% at room temperature is estimated takinginto consideration the ratio of the PL intensity at room temperature andlow temperature as indicated in FIG. 20.

The polarization characteristics of the semipolar QWs emission wereassessed by inserting a polarizer into the optical detection path of thePL setup. In contrast to the case for conventional c-plane material,polarized light emission is expected for group III-Nitrides grown onnon- and semipolar crystal orientations. As can be seen in FIG. 12strong light polarization was determined for the measured MQW sampleemitting at λ=237 nm. The integrated polarization is at least 35%, withthe spectral dependence shown in FIG. 21. As theoretically predicted thestronger emission was recorded for light that is linearly polarizedparallel to the c′-axis of the wurtzite crystal as shown in plot 2101,whereas the weaker emission originates from polarized lightperpendicular to the c-axis as shown in plot 2102.

A number of values and ranges are provided in various aspects of theimplementations described. These values and ranges are to be treated asexamples only, and are not intended to limit the scope of the claims.For example, embodiments described in this disclosure can be practicedthroughout the disclosed numerical ranges. In addition, a number ofmaterials are identified as suitable for various facets of theimplementations. These materials are to be treated as exemplary, and arenot intended to limit the scope of the claims.

The foregoing description of various embodiments has been presented forthe purposes of illustration and description and not limitation. Theembodiments disclosed are not intended to be exhaustive or to limit thepossible implementations to the embodiments disclosed. Manymodifications and variations are possible in radiation of the aboveteaching.

The invention claimed is:
 1. A method, comprising: slicing a bulk AlNboule to provide a substrate having a growth surface having a nonc-plane crystallographic orientation; epitaxially growing an epitaxialAlN layer having the non-c-plane crystallographic orientation on thegrowth surface; and epitaxially growing a III-N heterostructurecomprising an UV emitting active region, the heterostructure having thenon-c-plane crystallographic orientation on the epitaxial AlN layer, theUV emitting active region configured to emit UV radiation in response toelectron beam pumping of the active region.
 2. The method of claim 1,wherein epitaxially growing the heterostructure comprising the UVemitting active region comprises epitaxially growing multiple AlGaInNquantum wells having Al content greater than about 50%.
 3. The method ofclaim 1, wherein the non-c-plane crystallographic orientation is a(20-21) crystallographic orientation.
 4. The method of claim 1, whereinepitaxially growing the heterostructure comprises growing aheterostructure having a surface roughness less than about 1 nm for a 5μm×5 μm scan as measured by atomic force microscopy.
 5. The method ofclaim 1, wherein epitaxially growing the III-N heterostructurecomprising the UV emitting active region comprises epitaxially growingan AlGaInN active region including one or more quantum well structureswith Al content greater than about 50% having a non-c-planecrystallographic growth orientation.
 6. The method of claim 5, whereinthe quantum well structures have a semi-polar crystallographic growthorientation.
 7. The method of claim 6, wherein the semi-polarcrystallographic growth orientation is a (20-21) or (20-2-1)orientation.
 8. The method of claim 1, wherein the active region isconfigured to provide spontaneous or stimulated emission at wavelengthsless than 250 nm.
 9. The method of claim 1, wherein the active regionincludes about 1 to about 50 quantum wells having Al composition ofabout 70%.
 10. The method of claim 9, wherein the quantum wells of theactive region are separated by barrier layers having Al compositionbetween about 80% to about 99%.
 11. The method of claim 9, wherein athickness of each quantum well is in a range of about 0.5 nm to about 5nm.
 12. The method of claim 1, wherein a thickness of the active regionis in a range of about 200 nm to 1000 nm.
 13. The method of claim 1,wherein at least a portion of the heterostructure is n-doped.
 14. Themethod of claim 1, wherein a degree of polarization of UV radiationemitted by the active region is at least about 35%.
 15. The method ofclaim 1, further comprising thinning the sliced bulk AlN substrate. 16.A method, comprising: slicing a bulk AlN boule to provide a substratehaving a growth surface having a non c-plane crystallographicorientation; epitaxially growing an epitaxial AlN layer having thenon-c-plane crystallographic orientation on the growth surface; andepitaxially growing a III-N heterostructure comprising an UV emittingactive region, the heterostructure having the non-c-planecrystallographic orientation on the epitaxial AlN layer, the UV emittingactive region configured to emit UV radiation in response to electronbeam pumping of the active region; and forming a first reflector and asecond reflector, the active region disposed between the first reflectorand the second reflector.
 17. The method of claim 16, wherein: the firstreflector comprises an epitaxial distributed Bragg reflector (DBR)having a non-c-plane orientation; and the epitaxial heterostructure isdisposed on the first reflector.
 18. The method of claim 16, furthercomprising forming at least one contact electrically coupled to theheterostructure that provides a current path for discharging electronsarising from electron beam pumping of the heterostructure.
 19. Themethod of claim 18, wherein forming the contact includes forming thecontact having an aperture and wherein the second reflector is disposedwithin the aperture.
 20. The method of claim 18, wherein forming thesecond reflector comprises forming an epitaxial DBR disposed on theheterostructure.